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330                              CHEMICAL ENGINEERING
gravity of the gas or fluid. If s is zero (it will be negligible when representing the specific gravity of air) then D'/Z>" equals s"/Y- That is, the free settling ratio cannot be greater than s"/s'. The free settling ratio of galena on quartz in water is 4.01. In air it becomes 2.08. Since bodies fall faster in air, a much greater number of pulsions can be given in an air jig, and by this means, the rapidity of separation and capacity can be increased to a point where theoretically it compares favorably with jigging in water. The principal factor which militates against the employment of air jigs is the cost of compressing sufficient air to give capacities which will compare favorably with wet jigs. With the possible exception of the Plumb air jig which attained some prominence some years past the mechanical details of such air jigs which have been invented have been unsatisfactory. The advance of the ore entirely by gravity tends to reduce capacity. The great air pressure which would be required to lift other than sand sizes limits the air jig to these sizes. The dust which would be made in the fine grinding that this implies cannot at the present stage of the concentrating art be satisfactorily treated by dry methods so air separation methods as primary methods cannot usually be given much consideration in devising concentration schemes.
Harz Jigs and Jigging.—In discussing the screen and jig ratio previously in this section hindered settling ratios were given as governing the range of permissible screen size which should be fed to a jig. It must be plain that since these ratios apply to pure minerals much closer sizing must be employed if there is much disseminated material in the gangue or in other words there must be close sizing if a middling product is to be drawn from the jig as well as a tailing. If concentrates are made on the jig and no clean tailing can be made the question of close sizing is not then of very great importance. Of late years the tendency is away from close sizing and indeed away from jigging altogether. This trend is partly due to the cost for the screening equipment, its relatively large upkeep and the labor of attendance for it and jigs. Very skilled and conscientious labor is required for jig operation. None of these adverse factors would weigh if good screening obtained in the mills and the jig would effect the close separation that is desired. Good screening can be obtained in the mills but it is not often found. The reason it is not found is because the screens are too heavily loaded.
In the matter of the jig not removing all the middlings the fault is not in the principle employed for as will be shown a little later it is capable of effecting separations where there are only minute differences in specific gravity, but in the mechanical details of the jig. For coarse feeds the only practical means which have been devised for removal of concentrates and middlings is the top discharge in some form.
The only way these discharges can be operated successfully is to let the material in any compartment collect until it is very much richer than the material entering it. The material removed in any compartment must have greater average specific gravity than the material entering it. This mode of operation does very well so long as there is heavy material to be removed such as galena, blende, etc. but when the light middlings containing only small proportions of such substances are to be taken out the top discharge method fails. The top discharge consists of a pen which dips down into the bed and is supposed to exclude in withdrawal all except the lower rich layers. The exit from the pen is at a lower level than the surface in the beds outside the pen to provide for differences in specific gravity inside and outside the pen and thus to provide a velocity head for removing the rich lower layers. In the concentrate removal